The location of WLAN devices can be performed by various means. Of particular interest for this disclosure is the location of devices that are based upon the Institute of Electrical and Electronics Engineers (IEEE 802.11) technology, commonly known as Wi-Fi. Various methods can be used to locate an access point (AP), or a station (STA). These methods may be classified as active, passive and combined active and passive. In an active location scheme, a device that is determining the location or range, the measuring device, transmits certain packets to the device being located, the target device, and the common method is to measure the time of arrival (TOA) of the response from the target device and compare that to the time of departure (TOD) that the packet was transmitted by the measuring device. In a passive location scheme, a measuring device simply monitors the TOAs of non-stimulated transmissions from the target device.
In such location systems it is common to use multiple measuring devices to determine the location. In such a scheme simultaneous TOA and/or TOD measurements are taken by different measuring devices situated at different points and from these measurements the location of the target device is calculated. For example, in a passive location scheme, the TOA of a transmission from the target may be simultaneously received at several sites. The difference in the TOAs between two sites is known as the time difference of arrival (TDOA). The TDOA is related to the difference in path lengths between the target and each receiving site, and for each pair of receivers the TDOA results in a hyperbola along which the location of the target lies. The addition of a third site will provide a second hyperbola and the location of the target will be indicated by the interception of the two hyperbolas.